The present invention relates to an electron beam lithographic apparatus which describes a fine pattern on the surface of a semiconductor specimen by irradiating the surface of the specimen with an electron beam.
To meet the demand toward high-speed processing in recent years, technical development has been pushed forward in the field of electron beam lithographic apparatus to increase the electric current of beam and to increase the resist sensitivity in addition to adapting a variable shape beam system. As the resist becomes highly sensitive and the beam current increases, however, there arise problems in that the specimen is damaged to develop fog due to electrons that are reflected and scattered by the surface of the specimen and by the inner walls of the apparatus, and that the electron beam impinges upon the residual gases in the apparatus whereby the residual gases are polymerized and are deposited in the apparatus giving rise to the occurrence of contamination.
The inventors have forwarded the study in regard to damage caused by the specimen by the conventional electron beam lithographic apparatus based upon the variable beam system that is shown in FIG. 2 and contamination in the apparatus.
In FIG. 2, a first shaping iris 12 and a second shaping iris 14 are arranged under (in front of) an electron gun 10, and a shaping deflector 16 is disposed between the first shaping iris 12 and the second shaping iris 14. A blanking electrode 18 is provided under the second shaping iris 14 to bend a main beam 24 that has passed through an opening 20 of the first shaping iris 12 and an opening 22 of the second shaping iris 14. An objective iris 26 provided under the blanking electrode 18 has an opening 30 that guides the main beam 24 onto a specimen 28. A beam deflector 32 is provided under the objective iris 26 to scan the main beam 24. In FIG. 2, reference numeral 34 denotes a leakage beam, and 36 denotes a fog exposure on the surface of the specimen caused by the leakage beam 34.
According to the thus constructed electron beam lithographic apparatus of the variable shape beam system, the main beam 24 which has passed through the first shaping iris 12 is adjusted by the shaping deflector 16. That is, the shaping deflector 16 projects a projected image of the opening 20 of the first shaping iris 12 onto the opening 22 of the second shaping iris 14, to form a conjugate beam of the opening 20 and of the opening 22. The main beam 24 which is the conjugate beam passes through an opening 30 of the objective iris 26, converged by electron lenses 1, 2, 3 and 4, and reaches a specimen 28. The beam deflector 32 is served with a voltage controlled by a controller that is not shown in FIG. 2, whereby the main beam 24 scans along the surface of the specimen 28 to describe a fine image on the surface of the specimen 28.
Thus, as a line or an image is described on the specimen, the voltage applied to the blanking electrode 18 is changed, whereby the main beam 24 is deflected as shown in FIG. 2 and is cut by the objective iris 26. Then, new lithographic data are transferred to the controller, the voltage applied to the beam deflector 32 is controlled, the voltage applied to the blanking electrode 18 is returned to the initial value, and the next lithographic step is initiated.
As the main beam 24 passes through the first shaping iris 12, the second shaping iris 14 and the objective iris 26, however, the main beam is partly reflected and scattered by the edges of the openings 20, 22 and 30, resulting in the formation of a leakage beam 34. The leakage beam 34 is also converged on the surface of the specimen 28 owing to the optical image-forming conditions that are essentially the same as those for the main beam 24. Moreover, since the electrons that are reflected and scattered at the edges of the openings are not uniform in regard to their scattering conditions, angle of emission and kinetic energy at the edges, the leakage beam 34 partly falls on the specimen 28 to form fog, despite the main beam 24 is cut by the blanking electrode 18 as shown in FIG. 2. In this case, the major portion of the leakage beam 34 consists of electrons that are scattered at the edge of the opening 22 of the second shaping iris 14.
Fog exposure 36 due to the leakage beam 34 becomes conspicuous due to deposition of insulating material (contaminating substance) that stems from the residual gases when the electron beam is irradiated for an extended period of time, due to the electric field established by the accumulation of electric charges at the openings of the irises, due to the divergent angle of the main beam 24, and due to the dispersion of energy. Even if the leakage beam 34 may be about 1 ppm (1/10.sup.6) of the main beam 24 and the resist exposure by the main beam may be about 1 .mu.s, the resist is exposed if the specimen 28 is irradiated with the leakage beam 34 for one second. Therefore, if the lithographic data are transferred for extended periods of time, the specimen 28 is damaged due to fog exposure 36 caused by the leakage beam 34.
To cope with such a problem, it can be contrived to increase the amount of deflection at the time of blanking to decrease the amount of the leakage beam 34, i.e., to increase the voltage that is applied to the blanking electrode 18. With the blanking voltage being boosted, however, extended periods of time are required for the applied voltage to rise or break down, deteriorating the response characteristics.